The present invention, in some embodiments thereof, relates to polymeric material science and, more particularly, but not exclusively, to novel HIPE-derived shape-memory polymeric materials.
Smart materials constitute a class of substances exhibiting a change when exposed to an external stimulus. The change can be effected in a controlled and sometimes reversible fashion. The external stimuli can be physical or chemical and can include, for example, temperature, stress, moisture, pH, electric fields, and magnetic fields. A sub-class of smart materials includes shape memory alloys and polymers. Shape memory polymers (hereinafter SMPs) are polymeric smart materials that have the ability to return from a temporary deformed shape to their original shape, when the deforming stress has been ceased and the polymer has been exposed to an external trigger (stimulus), such as a temperature change. Deformation can be, for example, compression, corrugation, bending or folding. After a typical SMP undergoes deformation, it “remembers” its original shape when the deforming stress is removed and the recovery-triggering stimulus is activated (e.g., when a temperature change activates its “memory”).
Shape-memory polymers are useful in numerous applications spanning various areas of everyday life, such as, for example, smart fabrics, heat-shrinkable tubes for electronics or films for packaging, self-deployable sun sails in spacecraft, self-disassembling mobile-phones, and intelligent medical devices and implants for minimally invasive surgery.
SMPs have generated substantial interest for biomedical applications as they offer the ability to promote minimal-incision invasive surgery, provide structural support, exert stabilizing forces, elute therapeutic agents, and possibly biodegrade. In general, polymeric medical devices can be engineered to elicit a shape-memory effect, triggered by temperature, pH, humidity, light, electric or other stimuli of facilitating molecular motion and enabling shape recovery. SMPs bear most of their promise in their potential to provide compacted medical devices for minimally invasive surgery, which could be passed through a smaller incision in their temporary folded/shrunk shape and be deployed to their full original shape once inside the body [Yakacki C. M. et al., Adv Polym Sci, 2010, 226, 147-175].
Tobushi, H. et al. [Smart Materials & Structures, 2004, 13, 881-887] disclose shape-memory polyurethane foams whose recovery temperature is based on their glass transition temperature (Tg) ranging between 50 and 60° C., and having foam densities about 0.07 grams per centimeter cubed.
U.S. Pat. Nos. 5,049,591 and 6,583,194 teach shape-memory polyurethane foams which take on a deformed shape and an as-molded shape interchangeably at a temperature higher than the glass transition temperature (Tg) thereof.
U.S. Pat. No. 6,817,441 teaches a shape memory foam member which is compressible with heating; cooled with keeping the shape memory foam member in the compressed state; and released from the compressive pressure on the shape memory foam member after cooling, substantially recovering its original shape by heating.
U.S. Patent Application having Publication No. 2009/0149617 teaches shape memory polymer (SMP) networks formed using acrylate-based monomers comprising mono-functional acrylates which are controllably crosslinked using a crosslinker such as poly(ethylene glycol dimethacrylate) (PEGDMA).
U.S. Pat. No. 7,795,350 teaches shape memory polymeric materials having a glass transition temperature, Tg, exceeding room temperature and exhibiting a rubbery modulus and elasticity derived substantially from physical crosslinks. These materials are prepared by blending components including one crystalline polymer and two amorphous polymers.
The shape-memory effect is not an intrinsic property, meaning that polymers do not display this effect by themselves. Shape-memory results from a combination of polymer chemistry, polymer morphology, and specific processing and can be referred to as imbuing polymer with functionality. By conventional processing, e.g. templating, extruding and injection molding, the polymer is formed into its initial, permanent shape. Afterwards, in a process called programming, the polymer sample is deformed and fixed into the temporary shape. Upon application of an external stimulus, the polymer recovers its initial, permanent shape.
One aspect in polymer functionalization relating to shape-memory is crystallinity. As is widely known in the art, polymers are composed of long molecular chains which form irregular, entangled coils in the melt. Some polymers may retain such a disordered structure upon cooling, and thus convert into amorphous solids, while in other polymers, the backbone chains rearrange upon cooling and form partly ordered regions, referred to herein as crystalline regions. Polymers that can form crystalline or semi-crystalline regions upon cooling from the melt are referred to as “crystalline polymers” or “semi-crystalline polymers”.
Polymer crystallinity can play an important role in the shape-memory phenomena, wherein melting and freezing of the crystalline regions serve as molecular switches between the different shape states. The transition point is governed by the melting temperature, and heat serves as a stimulus.
FIG. 1A presents an illustration of the cycle of programming and recovery of a shape-memory polymer having a semi-crystalline morphology in at least the deformed shape. This cycle can be repeated several times, with different temporary shapes in any subsequent cycle. As can be seen in FIG. 1A, net-points, represented by black dots, which determine the permanent shape of the polymer network and can be of a chemical (covalent bonds) or physical (intermolecular interactions) nature, and crystalline backbone chain regions, represented by parallel lines, serve as molecular switches.
Shape-memory polyurethane foams whose recovery temperature is based on Tm of about 30° C., manufactured by the salt leaching method, having a cell size range of 400 to 1000 μm and a density of about 0.11 grams per centimeter cubed have been prepared and characterized previously [Chung, S. et al. Journal of Applied Polymer Science, 2010, 117, 2265-2271].
Crystallinity can also be conferred by side-chain moieties, as oppose to, or combined with, backbone chain crystallinity, as employed in other polymeric applications, such as disclosed in U.S. Pat. No. 3,853,778. Side-chain crystallinity has also been employed in shape-memory polymers, as disclosed for example in U.S. Pat. No. 5,888,650.
FIG. 1B presents an illustration of the melt-freeze cycle of a polymer having a semi-crystalline morphology conferred by side-chain moieties.
One particular polymer templating, relating to a processing alternative in imbuing polymer functionality, is known as solution or emulsion templating, as achieved in polymerization within some emulsions. Such processes are disclosed in, for example, U.S. Pat. No. 7,053,131.
High internal phase emulsions (HIPEs) are typically formed from two immiscible liquids, typically being water as a major dispersed or internal phase, and a highly hydrophobic liquid as a minor continuous or external phase, in the presence of a surfactant which is insoluble in the internal phase. The amount of surfactant needed to stabilize a major phase dispersed within a minor phase may reach up to 30% of the weight of the minor phase. HIPEs can also be stabilized through the formation of Pickering emulsions, as described below.
PolyHIPEs are highly porous polymers synthesized by polymerization of monomers within the external phase of HIPEs with internal phase volumes that are typically greater than 74% by volume of the emulsion. Most polyHIPEs are based on the co-polymerization of hydrophobic monomers and crosslinking co-monomers within the continuous phase of water-in-oil (w/o) HIPEs, followed by the removal of the internal phase, thereby producing a porous air-filled polymer.
A variety of polyHIPEs and polyHIPE-based materials have been synthesized and reported in the art. The porous morphology and properties of a polyHIPE was found to depend, among other factors, on the type and amount of the HIPE-stabilizing amphiphilic surfactant.
High internal phase emulsions stabilized by surfactants and polyHIPEs made therefrom are disclosed, for example, in U.S. Pat. No. 6,147,131, which teaches porous polymeric materials (foams) made from HIPEs which include water-in-oil high internal phase emulsions having at least 70% of an internal aqueous phase and less than 30% of an external oil phase, wherein the oil phase comprises a vinyl polymerizable monomer and a surfactant effective to stabilize the emulsion, and wherein the surfactants are oil soluble and include an oxyalkylene component.
Surfactant-based polyHIPEs are disclosed in, e.g., U.S. Pat. No. 3,988,508. Typically, the surfactants which are used in polyHIPEs are difficult and/or costly to remove. This disadvantage is more acute for polyHIPEs where unusually large quantities of surfactant are needed, hence displacing/replacing the surfactants in HIPEs can prove advantageous, especially for polyHIPE syntheses.
PolyHIPEs based on long side-chain acrylic monomers using divinylbenzene containing divinylbenzene (DVB) or ethylene glycol dimethacrylate (EGDMA) as comonomer crosslinking agents have been reported [Livshin, S. et al., Macromolecules 2007, 40, 6349-6354; and Macromolecules 2008, 41, 3930-3938], wherein the comonomer crosslinking agent was mixed with the other monomers in the polymerization reaction to afford a crosslinked copolymer. It would be expected that such polymers would possess some shape-memory due to the crystallizable side-chain moieties, however, these polyHIPEs were not reported as exhibiting any significant shape-memory attributes, probably due to the fact that copolymerization and crosslinking using a comonomer reduces crystallinity of the long side-chain moieties significantly, essentially by restricting the movement of the backbone chains as well as the side-chains [Livshin, S. et al., Soft Matter, 2008, 4, 1630-1638].
A Pickering emulsion (named after S. U. Pickering who first described the phenomenon in 1907) is a surfactant-free emulsion stabilized by micro- or nano-scaled solid particles that preferentially migrate to the interface between the two liquid phases. The aforementioned standard amphiphilic surfactants reduce the oil-water interfacial tension. The solid particles of a Pickering emulsion form rigid shells that surround polyhedral or spheroidal droplets of the dispersed phase and prevent coalescence thereof. The particles' shape and size, inter-particle interactions, and the wetting properties of the particles with respect to the liquid phases affect its ability to stabilize HIPEs. The stability of Pickering emulsions based on inorganic particles can be enhanced by chemically modifying the particles' surface with organic moieties that increase their tendency to migrate to the interface, and determines their ability to stabilize oil-in-water (o/w) or water-in-oil (w/o) emulsions.
Several different chemical surface modification methodologies, including silane modification, have been used to change the hydrophilic nature of the surface of silica nanoparticles such that they are able to stabilize Pickering emulsions. Silane coupling agents are commonly used to enhance fiber/matrix adhesion in polymer composites. Alkoxysilanes and chlorosilanes contain groups that bind covalently with silica through reaction with the hydroxyl groups on its surface. These silanes also contain hydrophobic organic groups that decrease surface hydrophilicity. Silane-modification thus enhances the amphiphilic character of the particles' surface, making it more suitable for Pickering emulsions and the corresponding HIPE stabilization. The extent of silica surface reaction with methyldichlorosilane was demonstrated to affect the degree of hydrophobicity and to determine whether it would stabilize an o/w or a w/o Pickering emulsion. In addition to controlling surface hydrophobicity, a silane that bears a vinyl group as part of the chemical surface modification can act as a monomer during a co-polymerization reaction.
Pickering HIPEs containing up to 92% internal phase, stabilized with 1-5% by weight of either titania or silica nanoparticles, whose surfaces were modified with oleic acid, have been reported [Menner, A. et al., Chemical Communications, 2007, 4274-4276; and Ikem, V. O. et al., Angewandte Chemie International Edition, 2008, 47, 8277-8279]. Similarly, partially oxidized carbon nanotubes were used to stabilize HIPEs containing up to 60% internal phase [Menner, A. et al., Langmuir, 2007, 23, 2398-2403] and poly(methyl methacrylate) microgel particles were used to stabilize HIPEs containing 50% internal phase [Colver, P. J.; Bon, S. A. F., Chemistry of Materials, 2007, 19, 1537-1539].
U.S. Pat. No. 6,353,037 discloses foams containing functionalized metal-oxide nanoparticles and methods of making the same.
Thus, the advantages of using Pickering HIPEs with a relatively small amount of nanoparticles for forming polyHIPEs include eliminating the need for standard surfactants, eliminating the need for procedures to remove such surfactants, and eliminating the problems associated with residual and leachable surfactants. Most of the polyHIPEs synthesized from such Pickering HIPEs exhibited relatively large voids (300 to 400 μm in diameter). Smaller voids of about 50 μm in diameter were observed when poly(styrene/methyl methacrylate/acrylic acid) particles were used to stabilize Pickering HIPE [Zhang, S.; Chen, J., Chemical Communications, 2009, 2217-2219]. PolyHIPEs from Pickering HIPEs do not usually exhibit the highly interconnected porous structures typical of conventional polyHIPEs but, rather, exhibit a somewhat interconnected structure.